ORIGINAL ARTICLE

Laser-induced agitation and cavitation from proprietaryhoneycomb tips for endodontic applications

Roy George & Keith Chan & Laurence James Walsh

Received: 15 May 2013 /Accepted: 3 February 2014# Springer-Verlag London 2014

Abstract Cavitation and agitation generated by lasers influid-filled root canals create fluid movement and shearstresses along the root canals walls, enhancing removal ofthe smear layer and biofilm. When used with sodiumhypochlorite and EDTA, laser activation of aqueous fluidscan increase the efficiency of debridement and disinfectionof root canals. However, the use of forward-firing laserfibers with such solutions poses a risk of driving fluidpast the root apex, which could cause postoperative com-plications. The purpose of this study was to evaluate themechanism of fluid agitation caused by a novel honey-comb tip. Glass capillary tubes filled with distilled waterwere used to replicate single-tooth root canals. A 980 nmpulsed diode laser was used with 200 μm diameter plaintips, tube-etched conical tips, and honeycomb tips. Torecord fluid movements, the tubes were backlit and im-aged using a digital camera attached to a microscope. Thehoneycomb tips generated agitation with fluid movementdirected onto the walls, while both the conventional plainfibers and the conical tips created fluid movement largelyin a forward direction. The use of honeycomb tips altersthe pattern of fluid agitation, and this laterally directedeffect might lower the risk of fluid extrusion beyond theapex.

Keywords Cavitation . Honeycomb tips . Modified laserfibers . Radial firing tips . Root canals . 980 nm lasers

Introduction

The primary goal of endodontic therapy is to eliminate micro-organisms from the root canal system. Smear layer, a layer ofdebris formed on the walls of the root canal created bymechanical preparation [1], has been the subject of consider-able controversy, both in endodontics and in restorative den-tistry. Overall, the removal of smear layer is strongly recom-mended by most authors [2–6]. Most clinical protocols targetboth organic and inorganic components of smear layer [7] anduse EDTA and sodium hypochlorite in combination to removeit [8, 9]. To enhance the action of EDTA in dissolving smearlayer, ultrasonic agitation has been used [6], relying on induc-tion of shockwaves in the fluid-filled canal environment.

Agitation of fluids through shockwaves with the goal ofenhanced smear layer removal can also be achieved usingpulsed lasers, namely erbium lasers (Er:YAG and Er,Cr:YSGG) [10, 11] and diode lasers [12]. Lasers may inducecavitation and agitation in endodontic irrigants such as EDTA,which then enhance the removal of smear layer and debris [10].Hydraulic stresses created in an aqueous solution will raise itsambient temperature and increase its penetration into any opentubules, enhancing its effects [10, 11]. For both 940 and 980 nmdiode lasers, the formation and collapse of steam-containingbubbles inside aqueous fluids in a capillary tube model hasbeen demonstrated [12]. The forced collapse of bubbles createsagitation and fluid movement, causing shear forces.

A potential problemwith any agitationmethod is the risk ofextruding the irrigant fluid beyond the confines of the rootcanal [13]. With laser-generated cavitation, the design of thetip influences the pattern of energy emitted and the extent ofsmear layer removal [10]. Conical tips have been shown to bemore effective at lateral dispersion of energy and smear layerremoval than plain forward-emitting tips [10]. Moreover, laserenergy that is directed at 90° onto canal walls may be betterable to cause ablation by photomechanical effects [14].

R. George (*)School of Dentistry and Oral Health, Griffith University, Gold Coast,Australiae-mail: drroygeorge@gmail.com

K. Chan : L. J. WalshSchool of Dentistry, The University of Queensland, Brisbane,Australia

Lasers Med SciDOI 10.1007/s10103-014-1539-y

To address the technical challenge of directing laser energyat 90° onto the walls of the root canals, special modificationsto tips for visible, near infrared, and middle infrared wave-lengths have been developed [15–17]. Recently, our group hasdemonstrated the use of near infrared lasers for generatingshockwaves in water-based irrigating fluids, using both 940and 980 nm diode lasers [10, 12], even though these wave-lengths are not as strongly absorbed into water as those frommiddle infrared erbium lasers. These longer diode laser wave-lengths are much more strongly absorbed than shorter nearinfrared wavelengths such as 810 and 830 nm. Diode lasersare of additional interest because of the accompanying effectsof photothermal disinfection and biostimulation, both ofwhich also require optimal energy distribution in the rootcanal [18].

The present study examined the characteristics of cavita-tion and agitation produced by a 980 nm diode laser whendelivered into water using either a plain fiber, or one of twomodified laser tips—a conical tip, or a tip with a honeycombsurface pattern which has been shown to give ideal lateralemission of near infrared laser energy [15, 16].

Material and methods

Laser systems

The original model of the SIROLaser diode laser system (Cat.No. 60 88 749, Sirona, Bensheim, Germany), which emitsenergy at 980 nm, was used for this study. A series of 200 μmplain forward-emitting fibers suitable for endodontics wereselected. The fibers were modified by tube etching to give 1-mm long conical ends, or conical ends with a surface honey-comb pattern, as described previously [15, 16, 19–21]. Thelaser panel settings used for the study were peak powers of 2and 3 W delivered in pulsed mode with 25 ms pulse durationswith a 50 % duty cycle at 20 Hz, for an exposure time of 20 s.These parameters were based on previously determined opti-mal pulse energies and frequencies [10]. The system wascalibrated according to the manufacturer’s instructions to con-firm that the laser emission from a plain-ended fiber waswithin 5 % of the panel setting.

Capillary tube model

A glass capillary tube model using tubes with external diam-eters of either1 or 2 mm was used with a stationary laser fiberto allow direct viewing of shockwaves in aqueous media [10].Capillary tubes (Hirschmann Laborgeräte, Germany) with alength of 15 mm and an internal diameter of either 0.8 mm or1.55 mm (volumes of 7.54 and 28.32 mm3, respectively) wereused with distilled water. The former volume of 7.54 mm3 iscomparable to the average volume of maxillary or mandibular

canine root canal systems, which is 8.66±3.66 mm3 [22]. Oneend of the tube was securely sealed with adhesive (Blu-Tac;Bostic, Sydney, Australia) and mounted on a template thatincluded a measuring ruler to achieve a standardized positionof the laser fiber. A stereo microscope (Olympus, Tokyo,Japan) fitted with a digital camera (CoolPix 4500; Nikon,Tokyo, Japan) at 20× optical magnification and was used torecord each experimental run at a frame capture rate of24 frames/s. All studies were performed at an ambient roomtemperature of 25 °C.

Before each experimental trial, distilled water was intro-duced into the capillary tube using a 25-gauge needle attachedto a 10 mL syringe. The tube was overfilled using a flushingaction to ensure that the entire volume of the tube had beenfilled, and no air bubbles were present. The distal end of thelaser fiber tip was then inserted into the capillary tube to apreset position 5 mm from the open end. The time taken fromthe commencement of lasing to the first sign of bubble for-mation and then peak bubble formation inside the tube wasrecorded up to a maximum of 20 s. The entire study wasrepeated eight times for reliability.

Data analysis

All raw video was converted into avi file format, and theframes were analyzed using Image J software (version 1.45,© Wayne Rasband, National Institute of Health, USA) todetermine the time needed for the formation of the initialand peak cavitation bubble. The size of the largest andsmallest bubble formations were also recorded, as well asany bubble formation which occurred behind and in front ofthe various tips. Finally, the pattern of bubble formationaround each tip was also recorded. All data were tested fornormality and then analyzed using a t test or ANOVA, withpost hoc tests using IBM® SPSS® version 21 statistical soft-ware package with a significance level of P<0.5.

Results

No bubbles or agitation occurred in tubes with an internaldiameter of 1.55 mm, and hence this group is not discussedfurther.

Bubble formation in water in the small-diameter tubes(internal diameter 0.8 mm) was observed with all fiber designsat power settings of 2 and 3 W. In contrast, no bubbles wereobserved for power levels of 1 or 1.25 W when these wereused in a pilot study.

The honeycomb tips generated bubbles with fluid move-ment directed onto the walls of the root canal. Both theconventional plain fibers and the conical tips created bubbleslargely in a forward direction; however, bubbles with theconical tip were also directed onto the walls of the root canal

Lasers Med Sci

behind the tip (Figs. 1 and 2). The conical fibers generatedbubbles in a flame-shaped pattern with the base at the tip andapex several millimeters ahead of the tip. An oval-shapedbubble pattern was seen behind the conical tip. The honey-comb tip, on the other hand, created bubbles along the lengthof the modified tip.

Surface area of cavitation bubbles

The conical fibers and honeycomb fibers produced a numberof large and microbubbles at 2 and 3 W (Fig. 3). However,with plain tips, larger bubbles were seen consistently at 2 and3 W (Fig. 4). There was a significant difference (P<0.001) inthe surface area of the major bubbles formed with plain tipswhen the power was increased from 2 to 3 W. A similarobservation was not noted for conical and honeycomb fibers.

Time for formation of cavitation

The time needed to initiate the first cavitation bubble wasdirectly proportional to the average power for plain, conicaland honeycomb tips, taking longer to form the initial bubblewith the plain tips than with the conical or honeycomb tips(Fig. 4). The time taken for the cavitation to reach its peak wasa few seconds after the first observation of bubble formation(Fig. 4). There was a significant effect on the time takenbetween initial and peak cavitation for both tip design andpower (Table 1).

Speed of fluid movement

With straight fibers used at an average power of 2W, the speedof fluid movement was 4.1 mm/s (SD=2.2), while at 3 W, itwas 5.3 mm/s (SD=2.2). In contrast with conical tips, thespeeds were 4.1 mm/s (SD=2.6) at 2 W and 4.9 mm/s (SD=1.5) at 3 W; these were not significantly different from those

seen with straight fibers. Because of the larger area of disper-sion of energy with the honeycomb tips, the speed of fluidmovement was significantly less than both plain forward-firing fiber tips and conical fiber tips (P<0.05), at only1.1 mm/s (SD=0.6) set at 2 W ( Fig. 5). At a power settingof 3 W, the honeycomb tips generated an average speed of1.5 mm/s (SD=0.7), which was significantly less than theplain and conical fiber tips at 3 W (P<0.01).

Discussion

Laser-induced bubble formation may assist in agitating fluidsin root canal therapy; however, for the optimal effect, a num-ber of factors are important—the speed of the fluid motiongenerated, the shape of the cavitation bubbles, and the time forthe process to occur once lasing has commenced. The resultsof the present study show that a 980 nm laser can generatebubbles when average powers are above 2 W within a capil-lary tube with an internal diameter of 0.8 mm. This is realisticgiven the dimensions of root canals in most teeth. For largervolumes, greater peak pulse powers would be required, as wefound the powers used in this study were insufficient togenerate bubbles and fluid motion in larger diameter capillarytubes (1.55 mm). This would be an issue in large canals suchas those in maxillary canine teeth.

The generation of motion in fluid is dependent on theoverall volume of fluid irradiated. In small spaces, such asthe mesial root canal of lower molar teeth, a lower peak powercould generate larger effects, as the volume of fluid is low. Arecent study from our group [23] evaluated the efficiency ofEDTA activation using a 940 nm laser in root canals preparedusing rotary files (80 mJ/pulse, 50 Hz, 6 cycles of 10 s) andshowed that this greatly enhanced smear layer removal whenthis was assessed using a validated quantitative image analysismethod. It is therefore expected that lasing EDTA with the

Fig. 1 Distance (in millimeters)of cavitation bubbles in relation tothe tip in a capillary tube with aninternal diameter of 1 mm. Barsshowmean values, and error barsindicate standard deviations

Lasers Med Sci

980 nm diode laser should also improve smear layer removal,and this will be the subject of future work.

Past studies of apical extrusion [11] have shown that whenmiddle infrared erbium lasers are used, the volume of fluidpushed beyond the apex is the same as when conventionalopen-ended needles are used. As the fluid movement gener-ated by the 980 nm laser is some 6,000-fold slower than thatgenerated by an erbium laser (4–5 mm/s vs. ~ 30 m/s), thepossibility of extrusion of fluids past the apex is low. Having aradial emitting fiber which generates slower apical fluidmovement lowers this possibility even further.

Because plain fibers generate shockwaves in a forwarddirection, they should be placed coronal to the area of the canalbeing treated. With conical fibers and honeycomb fibers, alarge proportion of energy is directed laterally onto the walls

of the root canal, giving a different pattern of shockwaves.Conical fibers and honeycomb fibers can be placed into theroot canal system and activated at various points workinglengths—a technique that may offer advantages if the samefiber is also used for feedback purposes, i.e., measuring wheth-er all bacterial debris has been removed [24]. Further studiesare needed to explore how much benefit in terms of greatersmear layer removal is gained from using diode lasers withconical or honeycomb fibers to agitate fluids in the root canal.

The time taken to initiate the first bubbles and cavitationand the time to reach maximal activity are additional points ofinterest. The peak level of shockwaves is often seen a fewseconds after lasing (Fig. 4), meaning, there is a short delayuntil agitation occurs and irrigation is enhanced. Conicalfibers take less time to initiate cavitation than plain fibers.This can be explained by convergence of laser energy at aconical tip. The settings used in the current study are unlikelyto pose a major issue in terms of thermal stress to periodontalstructures, as a previous study using the same laser parametersin roots (average 2.5 W and 20 Hz) did show significantelevations in external root temperature [25].

Fig. 2 Photographs showing the bubble formation and shockwave pat-terns a of plain forward-firing tip, b of conical tips, and c of honeycombfiber tip. Note the forward emission with the plain fiber, the divergentemission profile of the conical fiber, and the emission all along the lengthof the fiber tip for the honeycomb fiber tip

Fig. 3 Honeycomb fiber in use, showing formation of cavitation bub-bles. Frames a, b, c and d show bubbles formation following initiation ofbubbles at times 0, 400, 600, and 800 ms, respectively, (frames 1, 10, 15,and 20 at 25 frames/s). This sequence shows the formation of majorbubbles (MjB) and minor bubbles (MnB)

Fig. 4 Area of cavitation bubbles (in square millimeters) in relation to thedifferent combinations of average power and tip design. Major bubbles(MjB) and minor bubbles (MnB) are seen with all groups

Table 1 Time taken in seconds for the formation of initial and peakcavitational bubbles

Initial Peak Initialvs. peak

2 vs. 3W initial

2 vs. 3W peak

Plain 2 W 13.12 (2.04) 15.6 (0.84) ***

Plain 3 W 7.01 (0.79) 10.9 (0.18) *** *** ***

Conical 2 W 2.42 (0.39) 16.3 (1.14) ***

Conical 3 W 1.73 (0.34) 12.8 (0.97) *** ns ***

Honeycomb2 W

5 (0.5) 8.47 (0.2) ***

Honeycomb3 W

3.23 (0.4) 5.7 (0.2) *** ** ***

Data show means and standard deviations

ns not significant

*P<0.05;**P<0.01;***P<0.001

Lasers Med Sci

Conclusions

Modifying fluid dynamics in root canals is important inachieving effective removal of smear layer and debris fromthe walls of the complex root canal anatomy. Modifying thefiber optics used may be helpful in achieving these goals. Alateral emission (radial firing) design can optimize the patternof agitation of water-based fluids in the root canal system. Thenature of the effect varies with the diameter of the canal, andthus further work is needed to optimize the parameters interms of laser wavelength, fluid type, and tip design to estab-lish parameters which are both effective and safe for clinicalapplication.

Acknowledgments Two of the authors (RG and LJW) are named co-inventors of the honeycomb surface modification to optical fibers. Wethank Sirona for providing on loan the laser system used in the study.

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Fig. 5 Time taken for initial bubble formation (in seconds) in distilledwater, with conventional (straight firing) plain and modified (conical andhoneycomb fiber) tips in a capillary tube. Note that several seconds longer

are needed for shockwaves to reach their peak levels once cavitation hasbeen initiated

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